Download O A

158
Advances in Natural and Applied Sciences, 5(2): 158-165, 2011
ISSN 1995-0772
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLE
Experimental Works On Photocell Based Visible Light Communication For Audio
Signal Transmission Application
2
Mohammad Syuhaimi Ab-Rahman, Luqman 1Al-Hakim Azizan, 1Hadiguna and 1Mohammad
Hazwan Harun
1
Spectrum Technology Research Group (Spectech) Depart. Of Electrical, Electronics and System Engineering,
Faculty of Engineering and Built Environmental Malaysia.
2
Space Science Institute (ANGKASA) Universiti Kebangsaan Malaysia.
Mohammad Syuhaimi Ab-Rahman, Luqman Al-Hakim Azizan, Hadiguna and Mohammad Hazwan
Harun: Experimental Works On Photocell Based Visible Light Communication For Audio Signal
Transmission Application
ABSTRACT
Visible Light Communication is the new trend of wireless technology today due to the influence of the
green technology awareness. Towards this objective a self-powered optical-wireless data-transmission system
that uses solar cells and works under red-LED illumination is described. Solar cells have both power-generation
and photodetection functions. Therefore it is the best candidate for the green communication proposal in which
the system is batteryless and similar to what have been implemented in power over Ethernet (POE). In this
paper we proposed an experimental set up to a green communication via LED communication via indoor open
space using photovoltaic cell as receiver. The result shows the system works excellently for the low data rate
transmission such as audio based application. The application is widely used in area of advertisement,
entertainment, announcement and communication.
Key words:
Introduction
Visible Light Communication is the most advanced communication technology using visible light between
400THz and 700THz (http://en.wikipedia.org/wiki/Visible_Light_Communications). This latest technology use
light that are visible to human eye to transmit information (http://www.vlcc.net/e/e_about.html). Light is a part
of our lives as we are using throughout day and night. For example, there are many devices including lightings
in office, home, commercial display, traffic lights, electronic home appliances such as television computers and
etc. Recently, all the devices has been fabricated using LEDs (Light Emitting Diodes) because of LEDs have
low power consumption and long life span (http://www.marktechopto.com). LED has a special characteristic
to light on and off very fast (http://www.vlcc.net/e/e_about.html). With this characteristic, data could be
transmitted by lighting LED on and off at ultra speed. This findings will be developed to replace the current
communication technology using radio waves.
In the field of communication today, there’s no need to explain the useful of wireless communication using
radio waves. Although radio waves had been used widely around the world, due to weakness and limitation
researchers is exploring the visible lighting as a medium of communication in future. Research is focusing on
LEDs that are expected to serve in the next generation lamps. LEDs offers advantageous properties such as
brightness, reliability, lower power consumption and long lifetime (Komine, 2004). This devices are not only
used for illuminating rooms but also for an optical wireless communication system (Komine, 2004). This
system is expected to be the indoor communication of the next generation.
Corresponding Author:
Mohammad Syuhaimi Ab-Rahman, Institute of Space Science (ANGKASA) Universiti
Kebangsaan Malaysia 43600 UKM Bangi, Selangor, Malaysia.
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
159
Today, among all wireless communication schemes, transmission using radio waves (RF) or microwaves
has dominated. This domination was primarily due to by their availability of high-sensitivity receivers and the
ability to provide either broad either broad coverage at low frequencies or line-of-sight propagation at high
frequencies (Gareth Parry., 2007). However, RF can support only limited bandwidth because of restricted
spectrum availability and interference (Singh, 2002). The Radio Frequency Interference will thus degraded user
connectivity in terms of throughput, link quality and range (Far Point Group, 2010). Besides, position detection
with cellular telephones and car navigation systems that use radio waves is not very accurate, as there is some
difficulty in identifying the exact location of the information terminal
(http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920). This drawback becomes more pronounced
w h e n e q u i p me n t i s u s e d i n u n d e r g r o u n d s h o p p i n g ma l l s o r i n s i d e b u i l d i n g s
(http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920). As such, visible light in the electromagnetic
(EM) wave spectrum was considered to be a potential solution to combat the plight faced by RF
(http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920).
The purpose of this proposal is to develop a communication system for next generation using visible light.
This system will implement the application of photo cells as a photo detector to absorb the illumination of
LED and turn it into electricity. By using solar cells to transmit the signal, the overall system, cost reduction
could be reached at optimum level. Moreover the system will use LEDs as source of illuminations and as a
medium of indoor communications. This means that the LEDs have duo-function replacing the current lamps
for effective illuminations and radio waves as wireless communication medium.
Literature Analysis:
A fundamental analysis done by T. Komine and M. Nakagawa on visible light communication come to
conclusion that visible light communication is expected to be the indoor wireless communication of the next
generation due to the possibility of transmitting high data rate (Komine, 2004; Tanaka, 2001). In this paper
also they discussed about reflection and intersymbol interference. From the analysis, they found that
communication performance is degraded severely by intersymbol interference (Tanaka, 2001). Intersymbol
interference depended on the data rate and the Field Of Receiver (FOV) as the as the LED lights are
distributed within a room and the irradiance of light is wide for function of lighting equipment.
Deqiang, Xizheng and Linpeng had carried out the numerical simulation of optimal lights layout scheme
for visible light communication to find the effectiveness of this latest communication system. There are two
kinds of communication system being proposed, optical up-link and optical down-link (Deqiang, 2007). Up-link
has a small superficial area and narrow angle of irradiance like an electric torch whereby the LED will be
lighten up from the bottom to top. Generally the down-link system has a large superficial area and wide angle
of irradiance as the light will be located at the ceiling and lighten up from top to bottom. The performance
of visible light communication is analyzed using the optimal lights layout scheme in term of the received
power and bit error rate(BIT). Base on the numerical simulation results, the received power is very large
compared with the infrared wireless communication which will make broadband communication possible
(Deqiang, 2007). The effectiveness of the light layout is analyzed based on the received power. As the result
from the simulation of optimal lights layout scheme for visible light communication they came out to the
conclusion that this technique can be applied to obtain the excellent performance.
A study on challenges and possibilities of visible light communications done by O’Brien, Zeng, Le-Minh,
Faulkner and W.Walewski introduced the principles of VLC and outlines some of its major challenges and
overview of its application. In this paper there are few problems for implementation that was highlighted. The
problems that has been raised up are, the limited bandwidth of LED that will affect the high-speed
communication, low bandwidth of the transmitter, the disadvantage of transmitter equalization, providing an
uplink to the distributed transmitter structures can be problematic and visible light communication is subject
to regulation by non-communication standard (O'Brien, 2007).
Modulated Led:
Audio signals can be carried in radio waves through space and in electrical pulses through metal wires.
Other forms of electromagnetic radiation, including visible light can carry audio signals through optical
modulation. In modern communications systems such as long haul communication and high-bandwidth
communication lines, modulated signals carried on a beam of light is commonly used. The relatively high
frequencies of visual light can carry a lot more information than lower frequency radio waves. In optical fiber
communication systems often utilize semiconductor light sources such as laser diode (LD) or light emitting
diode (LED).
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
160
Both have the advantage that the digital modulation at logic circuit voltages can be directly applied to the
operating bias current of the diode, which in turn varies the output power
Intensity Modulation:
In optical communications, intensity modulation is a form of modulation in which the optical power output
of a source is varied in accordance with some characteristic of the modulating signal. In intensity modulation,
there are no discrete upper and lower sidebands in the usually understood sense of these terms, because present
optical sources lack sufficient coherence to produce them. The envelope of the modulated optical signal is an
analog of the modulating signal in the sense that the instantaneous power of the envelope is an analog of the
characteristic of interest in the modulating signal. Recovery of the modulating signal is by direct detection, not
heterodyning.
Fig. 1: Summary of direct and external modulation-link gains (Cox, 1997).
An important conclusion that can be drawn from the data shown in Figure 1 is that the maximum
bandwidth of direct modulation links is roughly 20 Ghz (Cox, 1997).
Photodetectors:
Photodetectors in use today are based on the p-i-n structure; consequently the emphasis will be on this
type of detector in the discussion below. Long wavelength-i.e., 1.3and 1.55 nm-avalanche photodiodes have
been developed, but they have approximately an order of magnitude lower avalanche gain at the same excess
noise factor than silicon avalanche photodiodes (Cox, 1997). As is evident from the summary of reported
results shown in Figure 2 (Shun Liu, 2007) photodetectors are available with 3-dB bandwidths which are more
than sufficient for most link applications. However, the highest bandwidths come at a penalty to the detector
responsivity. As will be seen below, the square of this parameter is a term in the equation for link gain. At
any optical wavelength, the ideal responsivity of any photodetector is simply the electron charge divided by
the photon energy (where is Planck’s constant and is the speed of light in a vacuum).
Fig. 2: Responsivities and 3-dB bandwidths of state-of-the-art photodetectors, as summarized by Cox, C. III
and Helkey, R (1997).
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
161
At 1.3 nm this works out to about 1.05 A/W. Therefore, at low frequencies, where some commercially
available 1.3- nm photodetectors have fiber-coupled responsivities of 0.9 A/W, this factor contributes only
about 1.3 dB to the link RF-to-RF loss (as shown on the right-hand axis of Figure 2), whereas at higher
frequencies a typical photodetector responsivity of 0.1 A/W would contribute 20.4 dB to the link loss. Another
photodetector tradeoff is between linearity and optical power. All IMDD links have a reason to operate at high
optical power. For direct modulation, the maximum modulation frequency of diode lasers is proportional to
the square root of optical power (Way., 1987).
For external modulation, the link gain is proportional to the square of optical power, as will be discussed
further on. Unfortunately, photo detector nonlinearities begin to dominate over modulation-device nonlinearities
at high optical power densities. Recent results suggest that photodiode nonlinearities can be reduced by
increasing the bias, since at high detector currents the series resistance and/or space-charge effects reduce the
fraction of the bias voltage which actually appears as a field across the junction at higher optical powers.
In this paper we focused on the experimental work of visible light communications using the Photocells.
From the experiment done, we found that photocell work as the photo detector that can change the light
illuminations into electricity and finally convert it into signal.
Methodology:
Transmitter And Receiver Circuit:
Basically a communication system could be explained through the Figure 3. As drawn by the this system
could be divided into two parts, one is the transmitter and the second is the receiver. Focusing on the
transmitter circuit, an input signal will be injected in a driver circuit and will be change to a light signal using
the light source (LED). Then the signal will be transmit through the fiber optic medium to the receiver. The
receiver basically consist of the photo detector, signal amplifier and signal restorer. The photo detector senses
the luminescent power falling upon it and converts the variation of this optical power into a correspondingly
varying electric current (Keiser, 2000). The optical signal is generally weakened and distorted when it emerges
from the end of the fiber, an amplification of the optical signal and signal restoration will done to get the
output signal. This is a basic diagram of the communication system using the fiber optic as a medium.
Fig. 3:
In this paper, some modification of the communication has been made through simple experimental works.
The circuit is simple and similar to what has been reported in
(http://www.exploratorium.edu/square_wheels/modulated_led,). The experiment was carried out using air as the
medium of transmission and the photocell as the photodetector. Back to the basic circuit, this system has two
parts, that is transmitter and receiver. Refer to the circuit Figure 4.
In this experiment, the transmitter part is built using 4 components. The first component, which is super
bright LED, is used as the light audio transmission modulation. LED work to modulate the amplitude of the
laser based on the amplitude of the audio signal that we are trying to transmit. Standard 3.5mm audio jack,
the audio signal, which is fed to the circuit, is sent to the 470-ohm resistor and 9-volt battery. The LED is
provided by a steady DC current by the battery and under the effect of the battery alone, the LED glows with
a fixed brightness. The resistor function is to limit the current so the LED does not burn out. The Figure 5
showed the experimental works.
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
162
Fig. 4: Pre-Amplifier circuit.
Fig. 5: The modulated signal is carried on a beam of light and transmit it to the photocell.
For the second part of this system, the receiver, it is built using solar cell and speaker connected. The
solar cell is connected to the radio in order to get the output signal. The experimental set up is shown in
Figure 6.
Fig. 6: The radio connected with the Led is the transmitter while the photocell and the speaker is the receiver.
Weak fluctuating radio signal is added to the constant signal from the battery after the audio jack is
connected to audio source. The LED still glows, but now it blinking in synchronizes with the audio signal,
as varies amount of current passing through it. Electrical signals is generated when the blinking light hitting
the solar cell. This signal varies in synchronization with the original audio signal. Then, the signals are fed
to the speaker, recreating the original sounds from the audio source. This means that the audio signal is
transmitted from the LED to the photocell through air medium.
Table 3.1: Technical data for LEDs.
Type
Colour
IFmax.
VFtyp.
Standard
Red
30mA
1.7V
Standard
Bright red
30mA
2.0V
Standard
Yellow
30mA
2.1V
Standard
Green
25mA
2.2V
High intensity
Blue
30mA
4.5V
Super bright
Red
30mA
1.85V
Low current
Red
30mA
1.7V
IF max:
Maximum forward current, forward
VFmax.
VRmax.
2.1V
5V
2.5V
5V
2.5V
5V
2.5V
5V
5.5V
5V
2.5V
5V
2.0V
5V
just means with the
Luminousintensity
Viewingangle
5mcd @ 10mA
60°
80mcd @ 10mA
60°
32mcd @ 10mA
60°
32mcd @ 10mA
60°
60mcd @ 20mA
50°
500mcd @ 20mA
60°
5mcd @ 2mA
60°
LED connected correctly.
Wavelength
660nm
625nm
590nm
565nm
430nm
660nm
625nm
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
VF typ.:
VF max.:
VR max.:
Luminous intensity:
Viewing angle:
Wavelength:
163
Typical forward voltage, VL in the LED resistor calculation. This is about 2V, except for blue and white LEDs
for which it is about 4V.
Maximum forward voltage.
Maximum reverse voltage.You can ignore this for LEDs connected the correct way round.
Brightness of the LED at the given current measured in millicandela (mcd)
Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°.
The peak wavelength of the light emitted, this determines the colour of the LED measured in nanometer (nm).
Results and discussion
The constructed first circuit was tested in the laboratory to analyze some characteristics designed circuit.
We were tested the circuit by varying 3 parameters to get the output to easily analyze the characteristics of
the circuit. Those three parameters are frequency, distance and resistance.
The obtained results are as shown below for three characteristics that we were carried out the tests. First,
the power received for variable frequencies were analysed. Figure 7 shows the relationship of frequency of
transmission signal and power launched by the LED. From the result and ploted graph we can conclude that
the power is increase if the frequency increase. The voltage is constant for this experiment which is 2.5 Volts.
The total power received are directly proportional to the input frequency. However the as we increase the input
frequency, the power received is began to be linear.
Besides that, voltage characteristics was analysed by varying the distance as shown in Figure 8. The output
voltage is decreased when the distance is increased. This shows that the voltage is inversely proportional to
the distance. The frequency was constant at 1kHz throughout the experiment to avoid any other effects on the
results. Even though the expected results were obtained but the received voltage level is in millivoltage which
is considered very low. The lower voltage due to the some unexpected noise which occured during the
experiment.
Finally, current versus resistance characteristics was tested by varying the resistance value. The resistance
value was controlled by using potentiometer which connected to the circuit as shown in the circuit description
part. As expected, the current value was reduced when the resistance value increased as shown in Figure 9.
Figure 10 shows the some captured signal during the experimental work by using oscilloscope.
Fig. 7: Power Versus Frequency Characteristics.
Fig. 8: Voltage Received For Variable Distance at 1 KHz transmission rate.
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
164
Fig. 9: Current Required For Variable Resistance.
Fig. 10: (a) Input signal (b) Output signal at 4kHz input signal (c) Received voltage at 1kHz frequency for
variable distance (c) F Received voltage at 2kHz frequency for variable distance.
Conclusion:
The wireless communication based on photovoltaic receiver has been demonstrated successfully. The result
shows the system works excellently for the low data rate transmission such as audio based application. The
application is widely used in area of advertisement, entertainment, announcement and communication. Although
there are many large bandwidth broadband application proposed for home and other small world application
such as Videoconferencing (110 kbps), E-Learning (110 kbps), Telemedicine (110 kbps), Video Telephony (70
kbps), Near Video on Demand (1 Mbps), Movies on Demand (1 Mbps), Audio on Demand (110 kbps),
Telegaming (40 kbps), Home Shopping (40 kbps) and other (SRC., 2006) but the short range communication
which less energy consumption and with high bandwidth offering is still required to increase the system
efficiency as well as for green awareness (Douseki, 2004).
References
Cox, C. III and R. Helkey, 1997. Techniques and Performance of Intensity-Modulation Direct-Detection Analog
Optical Links. IEEE Transactions on Microwave theory and techniques 45(8).
Deqiang, D. K. Xizheng, X. Linpeng, 2007. “An Optical Lights layout Scheme for Visible-Light
Communication ”,Electronic Measurement and instruments, 2007. ICEMI ’07.8th International Conference
pp: 2-189(2): 194.
Douseki, T., 2004. “A batteryless optical-wireless system with white-LED illumination, “ Proc. Of the 15th
IEEE International Symposium on Personal, Indoor and Mobile radio Communicatios, 4: 2529-2533.
Far Point Group 2010. “The Invisible Threat: Interference and Wireless LANs”.
Gareth Parry and Paul Stavrinou, 2007. “Nature Photonics-Dominic O’Brien”.
Adv. in Nat. Appl. Sci., 5(2): 158-165, 2011
165
http://en.wikipedia.org/wiki/Visible_Light_Communications
http://www.vlcc.net/e/e_about.html
http://www.marktechopto.com/products/Application-Of-LEDs/consumer-electronics-and-general-indication-ofleds.cfm
http://www.cdrinfo.com/sections/news/details.aspx?newsid=10920
http://www.exploratorium.edu/square_wheels/modulated_led,
Komine, T., M. Nakagawa, 2004. “Fundamental analysis for visible light communication system using LED
Lights”.Consumer Electronics,IEEE, 50(1): 100-107.
Keiser, G., 2000. "Optical Fiber Communication," Mc Graw-Hill Education.
O’Brien, D.C., L. Zeng, H. Le-Minh, G. Faulkner, J.W. Walewski, S. Randel, 2007.“ Visible Light
Communications: challenges and possibilities”,Electronic Measurement and instruments. ICEMI ’07.8th
International Conference pp: 2-189(2): 194.
Singh, C., J. John, Y.N. Singh, K.K. Tripathi, 2002. “A Review on Indoor Optical Wireless Systems”, -IETE
Technical Review.
Shun, Liu, AtsuhiMinato., 2007. A New Lighting for Communication System for audio signal for white LED.
J. Light and Vis. Env., 31(2): 65-69.
S.R.C., 2006. Broadband Applications. http://www.connectingcommunities.info/article.cfm?id=211
Tanaka, Y., T. Komine, S. Haruyama and M. Nakagawa, 2001. “Indoor visible communication utilizing plural
white LEDs as lighting ,” Proc. Of the 12th IEEE International Symposium on Personal, Indoor and Mobile
radio Communicatios, 2: 81-85.
Way., W., 1987. Large signal nonlinear distortion prediction for a singlemode laser diode under microwave
intensity modulation. J. Lightwave Technology LT-5: 305-315.